1. Field of the Invention
The present invention relates to a driving apparatus operating in successive steps, comprising two clamping assemblies which are located in alignment with each other, each clamping assembly comprising a tubular body having an inner surface which is inclined with respect to the longitudinal axis of said tubular body, and a set of clamping members located in circumferentially spaced relation within the tubular body and having an outer surface which has the same slope as the inner surface of said tubular body, so that a relative axial displacement of the set of clamping members in either one direction or the other with respect to the tubular body produces a clamping or unclamping action, respectively on a plurality of cables whith which the driving apparatus is intended to cooperate in order to displace a load, each cable extending in a direction parallel to the axis of the tubular body and passing between two adjacent clamping members of each of the two sets of clamping members, and at least one hydraulic jack coupled to at least one of the two clamping assemblies in order to produce an axial displacement thereof.
2. Description of the Prior Art
The invention is more particularly concerned with a driving apparatus of the type described in French Pat. No. Fr-2,063,477. (U.S. Pat. No. 3,653,634). As shown in particular in FIG. 1 of the cited patent, the outer surfaces of all the clamping members 15 or so-called keys of the two clamping assemblies 11 and 13 all converge in the same direction F.sub.1. The operation of this known driving apparatus does not usually present any problem when it is necessary to exert a tractive force on a set of cables 5 in a direction F.sub.2 opposite to the direction F.sub.1 as is the case with prestressing of a concrete beam. It is an entirely different matter when a load has to be displaced both in the direction of the arrow F.sub.2 and in the direction of the arrow F.sub.1 in order to raise and lower said load, for example. In fact, since the driving apparatus is used in that case in a vertical position and the downward displacement of the load takes place in the direction of the arrow F.sub.1, when the piston 12 of the hydraulic jack and the clamping assembly 13, which is fixed to the tubular piston rod 14 of said jack and holds the cables 5, are allowed or caused to move all together in this direction with a view to lower the load, the clamping members 15 of the clamping assembly 11 have a tendency, under the action of their own weight and under the action of the friction forces exerted thereon by the cables 5, to move also in the direction of the arrow F.sub.1 with respect to the tubular body 11a of the clamping assembly 11 which is fixed. Therefore, the clamping members 15 of the clamping assembly 11 have a tendency to clamp and actually do clamp the cables 5, thus impeding any displacement of said cables 5 and of the load attached thereto in the direction F.sub.1. Furthermore, when the piston 12 is displaced in the aforesaid direction F.sub.1 and is accompanied in this movement by the tubular body 13a of the clamping device 13, the friction forces set up between the cables 5 and the clamping members 15 of the clamping device 13 are liable to cause a relative displacement of said clamping members 15 with respect to the body 13a in the direction of the arrow F.sub.2 and consequently to cause unclamping of the cables 5 which are no longer retained by the clamping device 13 in such a case.